Method of thermal stimulation of oil fields



United States Patent 3,285,336 METHOD OF THERMAL STIMULATION OF OILFIELDS Gerald H. F. Gardner, Pittsburgh, Pa., assignor to Gulf Research& Development Company, Pittsburgh, Pa., a

corporation of Delaware No Drawing. Filed Sept. 15, 1964, Ser. No.396,708

6 Claims. (Cl. 166-2) This invention relates to a method for recoveringpetroleum from underground reservoirs by the thermal stimulation of thereservoir, and more particularly it relates to a thermal stimulationprocess characterized by the broad distribution of heat in the interiorregions of a reservoir.

Many oil containing reservoirs have been discovered in which the oil istoo viscous to be recovered by primary recovery techniques. The futurediscovery of many additional reservoirs possessing this characteristicis a certainty. When oil recovery from reservoirs of this type isdesired, well stimulation by some appropriate technique is necessary inorder to produce the viscous oil in economic amounts. The application ofheat -by one of various methods is the most commonly used stimulationtechnique for viscosity reduction of the reservoir oil. The injection ofhot fluids such as steam, hot water and hot gases directly into theformation has been used. However, the heat cannot be transported morethan a relatively short distance from the well bore when this heatedfluid is injected into the formation. Although ordinary forward drive insitu combustion performs satisfactorily in some reservoirs, it usuallycannot be employed to recover viscous, immobile oils.

In an effort to get heat into the interior of oil-bearing formationscontaining viscous oil it has been proposed that one or more fracturesextending deeply into the formation be formed and propped. Thesefractures are then used as conduits for conducting heated fluidslaterally into the formation or they are used as the zone for in situcombustion. This approach offers the disadvantage that fracturing is anexpensive procedure and cannot be used in many formations which possesslithologic characteristics which resist fracturing.

According to my invention the oil bearing reservoir is heated by in situcombustion over a broad zone deeply into a formation without requiringfracturing. While hot fluid injection may not penetrate in someinstances further than 10 feet from a well bore, in accordance with myinvention the formation may be heated in a zone ranging as much as 10 to200 feet from the well bore. My process possesses an added advantagethat in situ combustion of the cheap reservoir oil is utilized as asource of heat as compared with the relatively expensive above groundheating of an auxiliary injection fluid such as steam. Although theheated zone resulting from my method is broad and extensive; it does notinclude the input well thereby protecting the well casing and equipmentagainst heat damage.

In accordance with my invention a well, which is nonproductive byordinary primary recovery methods but which contains an appreciableamount of viscous oil, is made to produce the oil. Once it isascertained that the reservoir contains recoverable amounts of a viscousoil my process is put to practice. A relatively small quantity of aliquid of low viscosity which is miscible with the reservoir oil, suchas liquefied petroleum gas, is injected as a slug into the formationadjacent to the well bore driving the reservoir oil ahead of it awayfrom the well bore. This is followed by the injection of an oxidizinggas such as air at a high pressure which is insufficient to fracture theformation. The miscible slug functions as 3,285,336 Patented Nov. 15,1966 ice a barrier separating the oxidizing gas from the oil. Since thelow viscosity liquid and the high viscosity oil exhibit a high mobilityratio, the driving gas will finger deeply into the formation in a greatmultiple of fingers driving the low viscosity liquid ahead of it anddisplacing the oil in these fingers. As the fingers move deeply into theformation the low viscosity liquid occupies a layer of decreasingthickness until it is effectively dissipated by thinning out andintermixing with the oil permitting the oxidizing gas to contact thereservoir oil over a broad front along the peripheries of the fingers.Continuing the injection of oxidizing gas now in direct contact with thereservoir oil over this broad front initiates and maintains combustionof the oil resulting in a progressive heating of the interior of thereservoir. Since the miscible fluid drives the reservoir oil away fromthe input well, there is insuflicient oil immediately adjacent the inputwell to support in situ combustion in this region.

After heating of the reservoir in this manner has progressed for asufficient time measured in days or Weeks to heat a recoverable quantityof oil, the injection of oxidizing gas is stopped and oil is producedfrom the injection well. A further advantage of my process is that theoil possesses a lowered viscosity as a result not only of the heatingbut in addition as a result of its admixture with the low viscosityfluid. This fluidized oil flows to the well as the result of any naturaldrive existing in the reservoir and by the combined effect of gravityflow and the driving force of the combustion gases which are confined inthe reservoir under pressure as well as from the pressure exerted by anyvaporized hydrocarbons. The well is produced until the flow of oil hasreduced to an economic minimum. When this results, injection ofoxidizing gas is repeated. If the fingers have become plugged with highviscosity reservoir oil, the injection of a slug of miscible. fluid isrepeated to re-establish the fingers. However, if the integrity of thefingers has been maintained further use of a miscible fluid is notrequired.

The use of miscible drive has previously been considered for therecovery of reservoir oil by secondary recovery techniques. It has beenrecognized that a miscible fluid has the tendency to finger into theformation particularly in situations in which high mobility ratios areinvolved. Much work has been directed towards the attainment of miscibledrive in a radial front. By my invention the tendency of the misciblefluid to finger through the formation is used to advantage and, in fact,this tendency, hitherto considered to be undesirable, is enhanced byutilizing miscible liquids which provide large mobility ratios.

A specific use of my process is now described. An exploratory well haslocated a ten foot thick oil-bearing formation at a depth of 2,000 feet.No natural drive is present. A core sample of the formation is obtainedand the oil and formation characteristics are analyzed above ground in alaboratory. The oil is ascertained to be non-flowable having a viscosityof about 1,000 centi poises at the reservoir temperature of 200 F. It isfurther determined that this oil is amenable to self-ignition in thepresence of oxygen in accordance with in situ combustion techniques. Theformation is also found to be well consolidated and to have suitablepermeability characteristics. It is concluded that the oil can berecovered by my process.

Open hole completion is used since the formation was discovered to besuficiently consolidated. Two thousand barrels of liquefied petroleumgas are injected into the formation over a period of three days. This isfollowed by the injection of air at a pressure of 1,500 p.s.i. This airinjectionis continued for about a week until about ten million s.c.f. ofair has been injected. At this point extensive fingering has occurred inthe reservoir to a radius approximately 100 feet from the well bore andcombustion has taken place over a broad front. Air injection is stoppedand the well is allowed to produce at a rate in excess of 100-barrelsper day. This oil is pumped to the surface for recovery of the liquefiedpetroleum gas and subsequent refining. When oil production reduces toless than barrels per day, air injection is renewed and the cycle isrepeated until the well become economically unproductive.

The air is injected in this process at a flux as high as possiblewithout fracturing the formation. A fracture would provide a path ofvery high permeability and would prevent the uniform fingering of thesurrounding formation. Air injection is stopped after a predeterminedamount has been injected or after the reservoir pressure exceeds apredetermined amount to avoid excessive compressor costs. The amount ofair that is injected is not critical but rather determines the extent ofthe formation that is heated and the amount of oil that can be recoveredin the production phase. However, air injection should be discontinuedand the well put into production before the heat loss to adjacentnon-productive formations becomes excessive and preferably well beforethis heat loss approaches the amount of heat being generated.

It has been determined that at least one hundred fifty barrels ofmiscible liquid should be injected for each foot of formation thicknessin order to develop a good fingering pattern. It has also beendetermined for optimum results that the injected miscible fluid shouldamount to about three to six percent of the oil in place in the volumeto be fingered and heated for oil recovery. If the oil is produced inabout twice the volume of the injected miscible liquid, the flowchannels will remain open and a second slug of miscible liquid will notbe required when the cycle is repeated. However, it is generallypreferred to produce the oil as long as it is flowing in suitablevolume.

By this process it is possible to heat and recover viscous oil up to 200feet or more from a well bore. Recovery of the heated, mobilized oil maybe from the injection well by a cyclic process as described or it may beproduced into one or more neighboring wells if they are sufficientlyclose to the injection well. In this latter instance air injection iscontinued until combustion breakthrough into the production well isimminent. In either instance in situ combustion and resultant heatingoccurs over a broad volume in the plurality of fingers formed by thismethod.

It is evident that utilization of this process requires an oil which isself-ignitable in the formation in the presence of an oxidizing gaswhich is injected under pressure. Furthermore, the formation shouldpossess suflicient permeability that the oxidizing gas and misciblefluid can be driven into the formation. The process can be utilized inboth open wells or cased perforated wells with single point or multiplepoint perforations.

Either liquefied petroleum gas or a similar light petroleum fraction ispreferred as the miscible liquid. However, other low viscosity organicliquids including halohated and oxygenated hydrocarbons, such as carbontetrachloride and lower alcohols, can be used either alone (pr-admixedwith light petroleum fractions. It is important that. themiscible liquidis not selfignitable in the presence xyge'n and that it have asufliciently low viscosity that i e fingering will occur. It ispreferred that the b lty ratio of the miscible fluid and reservoir oilbe at ast 100 to 1 inorder to accomplish superior fingering.

viously indicated the miscible liquid may be rehe. product oil forreuse. redoxidizing gas for economic reasons. izing' g-as may be used inaccordance with accepted knowledge in situ combustion technology. Forexample, oxygen enriched air oxygen itself may be utilized. Furthermore,other oxidizing gases may be added to the air to enrich it. Theparticular oxidizing gas used is not critical but rather determines thetotal volume of gas as a function of its usable oxygen content whichmust be injected to accomplish a specific amount of reservoir heating.

By this invention a process is provided for the recovery of viscousn-on-fiowable oils in an extensive volume of the reservoir Without therelatively expensive procedure of formation fracturing and without beingrestricted to shallow formation heating which is the result of hot fluidinjection methods. It is particularly suite-d for viscous oils having aviscosity greater than 500 centiipoises at reservoir conditions inreservoirs which possess some permeability with insuflicient naturaldrive. It combines the concurrent heating of a broad volume of thereservoir with the production of flow channels for the oil which 1. Anin situ combustion process for recovering oil.

from an underground oil-bearing formation penetrated by at least onewell bore characterized by the broad distribution of heat in theinterior regions of the reservoir which comprises the steps, injecting aslug of a low viscosity, oil-miscible liquid into the formation fromaninput well, said oil-miscible liquid being not self-ignitable in thepresence of oxygen, injecting an oxidizing gas into the well at asufliciently high pressure that it will drive the miscible liquid andthe oil ahead of it to form a great multiple of fingers projectingdeeply into the formation with the miscible liquid forming a separatinglayer between the oxidizing gas and the oil, injecting additionaloxidizing gas under high pressure to cause the miscible liquid layerto-move further from the well, whereby said layer is thinned out anddissipated and whereby the oil in the formation contacts said gas tocause the oil to 'be ignited along the peripheries of the fingers,maintaining the injection of oxidizing gas at a high rate to cause theconcurrent combustion of the ignited oil over a wide radial front toheat a substantial volume of the reservoir, and producing the mobilizedoil from said formation at least partially through said fingers.

2. A process in accordance with the claim 1 in which the mobility ratioof the miscible liquid and oil is at least to 1 and the oxidizing gas isair.

3. A process in accordance with claim 2 in which the miscible liquid isa hydrocarbon having from three to eight carbon atoms.

4. A process in accordance with claim 2 in which said miscible liquid isliquefied petroleum gas.

5. A process in accordance with claim 1 in which the oil is producedfrom the injection well.

6. A process in accordance with claim 1 in which the oil is producedfrom at least one well other than the injection well.

References Cited by the Examiner UNITED STATES PATENTS 3,129,757 4/1964Sharp l66-ll 3,134,435 5/1964 Wyllie l6625 3,167,121 l/l965 Sharp l66-llCHARLES E. O'C-ONNELL, Primary Examiner.

S. J. NOVOSAD, Assistant Examiner,

1. AN IN SITU COMBUSTION PROCESS FOR RECOVERING OIL FROM AN UNDERGROUNDOIL-BEARING FORMATION PENETRATED BY AT LEAST ONE WELL BORE CHARACTERIZEDBY THE BROAD DISTRIBUTION OF HEAT IN THE INTERIOR REGIONS OF THERESERVOIR WHICH COMPRISES THE STEPS, INJECTING A SLUG OF A LOWVISCOSITY, OIL-MISCIBLE LIQUID INTO THE FORMATION FROM AN INPUT WELL,SAID OIL-MISCIBLE LIQUID BEING NOT SELF-IGNITABLE IN THE PRESENCE OFOXYGEN, INJECTING AN OXIDIZING GAS INTO THE WELL AT A SUFFICIENTLY HIGHPRESSURE THAT IT WILL DIRVE THE MISCIBLE LIQUID AND THE OIL AHEAD OF ITTO FORM A GREAT MULTIPLE OF FINGERS PROJECTING DEEPLY INTO THE FORMATIONWITH THE MISCIBLE LIQUID FORMING A SEPARATING LAYER BETWEEN THEOXIDIZING GAS AND THE OIL, INJECTING ADDITIONAL OXIDIZING GAS UNDER HIGHPRESSURE TO CAUSE THE MISCIBLE LIQUID LAYER TO MOVE FURTHER FROM THEWELL, WHEREBY SAID LAYER IS THINNED OUT AND DISSIPATED AND WHEREBY THEOIL IN THE FORMATION CONTACTS SAID GAS TO CAUSE THE OIL TO BE IGNITEDALONG THE PERIPHHERIES OF THE FINGERS, MAINTAINING THE INJECTION OFOXIDIZING GAS AT A HIGH RATE TO CAUSE THE CONCURRENT COMBUSTION OF THEIGNITED OIL OVER A WIDE RADIAL FROM TO HEAT A SUBSTANTIAL VOLUME OF THERESERVOIR, AND PRODUCING THE MOBILIZED OIL FROM SAID FORMATION AT LEASTPARTIALLY THROUGH SAID FINGERS.